US3727127A

US3727127A - High sensitivity electron beam magnetometer

Info

Publication number: US3727127A
Application number: US00109240A
Authority: US
Inventors: H Heidenwolf
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1971-01-25
Filing date: 1971-01-25
Publication date: 1973-04-10
Anticipated expiration: 1990-04-10

Abstract

An electron beam magnetometer having a sensitivity equal to 10 9 gauss or better comprising an evacuated envelope, means for generating a stream of electrons positioned in the interior thereof, control grid and focusing electrode means positioned downstream along the axis of the generated electrons, a pair of cores of a ferromagnetic material for densifying an induced magnetic flux positioned downstream of the focusing electrode means so as to form an air gap for the passage of the electron beam, deflection means located downstream of the magnetic flux densifying material, target means connected to suitable indicating means located downstream of the deflection means and final acceleration anode means located adjacent the target means. All of said components are operably connected to a suitable power source.

Description

United States Patent Heidenwolf 1 Apr. 10, 1973 HIGH SENSITIVITY ELECTRON BEAM OTHER PUBLICATIONS MAGNETOMETER Electronics; Apr. 3, 1967; pp. 270-271 [75] Inventor: Hermann Heidenwolf, Adnet, Kliever t AIEE Trans 1947; pp- Austria [73] Assignee: The United States of America as Primary Examiner A1fred E Smith represented y the Secretary of the AttorneyHarry M. Saragovitz, Edward J. Kelly and Amy Herbert Berl [22] Filed: Jan. 25, 1971 [57] ABSTRACT [21] Appl' No" 109,240 An electron beam magnetometer having a sensitivity equal to 10' gauss or better comprising an evacuated [52] US. Cl. ..324/44 envelope, means for generating a stream of electrons [51] Int.Cl. ..G01r 33/02 positioned in the interior thereof, control grid and [58] Field of Search ..324/44; 313/89, 72 focusing electrode m n positioned wn ream along the axis of the generated electrons, a pair of [56] References Cited cores of a ferromagnetic material for densifying an induced magnetic flux positioned downstream of the UNITED STATES PATENTS focusing electrode means so as to form an air gap for the passage of the electron beam, deflection means 3,657,643 4/1972 Nicholson ..324/44 located downstremn of the magnetic flux densifying 2358901 9/1944 Ziebolz "'"324/44 x material, target means connected to suitable indicat- 2465277 3/1949 Schafeh" "313/89 X ing. means located downstream of the deflection 2562696 7/1951 X means and final acceleration anode means located ad- 2,720,558 10/1955 Skellett ..324/44 jacent the target means. All of said components are operably connected to a suitable power source.

7 Claims, 6 Drawing Figures HIGH SENSITIVITY ELECTRON BEAM MAGNETOMETER This invention relates to improvements in electron beam systems for responding to magnetic fields and more particularly to a high sensitivity electron beam magnetometer.

In many instances, it is desired to produce an electrical signal or impulse in accordance with relatively small changes in magnetic fields. For example, in detecting submarines from ships or airplanes, a device is desired which will produce a usable electric signal on the rather small change in local magnetic fields due to the relatively distant mass of the submarine. A similar need arises in regard to military mines intended to set off explosives on approach of a vehicle or ship within a predetermined distance. Similar problems arise in the investigation of magnetic fields of metallic or non-magnetic specimens of the earths field as in prospecting. In the measurement of magnetic gradient, an instrument sensitive to differences of magnetic fields is required.

The accuracy obtained to date with magnetometer systems such as the Foerster probe or the Rubidium vapor magnetometer is not sufficient to satisfy progressing demands of modern magnetometry. For example, any magnetic fields of the moon have as yet not been detected, although the earths magnetic field should still have some small effect there.

The present system operates on the principle that when a bar of a ferromagnetic material is positioned in a magnetic field, a magnetic flux is induced into the material. In the present system a ferromagnetic material of high permeability is encompassed in a vacuum tube downstream of a cathode. Inoperation, generated electrons are accelerated and impinged upon target means. The generated electrons are bent by the magnetic flux induced into the ferromagnetic material. In the absence of a local magnetic field, the bent and accelerated electron beam is adjusted so that it impinges equally on the target means by the application of a potential to defecting means. This gives a zero reading on a suitably calibrated indicating means. In the presence of a local magnetic field the generated electron beam impinges unequally on the target means. This gives a reading on suitable calibrated means indicative of the local magnetic field.

It is an object of this invention to provide and disclose an improved system adapted to respond to the gradient of a magnetic field.

It is a further object of this invention to provide and disclose an improved high sensitivity electron beam magnetometer having a sensitivity equal to gauss or better.

It is a further object of this invention to provide and disclose an improved electron beam magnetometer wherein the sensitivity thereof is enhanced by a magnetic field concentrator in the vicinity of the cathode.

It is a further object of the invention to provide and disclose an improved electron beam magnetometer comprising a magnetic field concentrator in the vicinity of the cathode which deflects the electrons, thus shifting the locating of deflection being originally focused on both lenticular plates equally to prevalently one or the other of said lenticular plates.

It is a further object of this invention to provide and disclose an improved electron beam magnetometer comprising an electron beam accelerator thereby necessitating higher voltage application on the deflection means to deflect the electron beam, thus increasing sensitivity.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawing in which:

FIG. 1 shows a cross-sectional elevated view, partially in schematic, of the present apparatus.

FIG. 2 is a section on line 33 of FIG. I.

FIG. 3 shows an alternative of FIG. 1 which comprises electromagnetic means to deflect the electronic beam.

FIG. 4 shows an alternative of FIG. 1 which comprises magnetostatic means to deflect the electron beam.

FIG. 5 shows a schematic illustration of an alternative of FIG. 1 which utilizes a Thermistor bridge to indicate the presence of the local magnetic field.

FIG. 6 is a schematic illustration of an alternative of FIG. 1 wherein the target means comprise a pair of thermocouples.

Referring now to FIG. 1 of the drawing, the system comprises elongated tubular envelope l1. Said envelope may be constructed of glass or any other suitable material. Cathode l3 and cathode shield 15 are positioned in the interior of envelope 11. Control grid means 17 is positioned adjacent cathode shield 15. First anode and electronic lens 19, which constitutes the focusing electrode, is positioned adjacent control grid 17 along the axis of tube 11. Upper pole shoe 21 and lower pole shoe 23 constructed of high permeability ferromagnetic materials are positioned adjacent focusing electrode 19.

Cores

21 and 23 comprise the internal magnetic flux densifiers. Permalloy, a material composed of about 78 percent nickel and 21 percent iron, was utilized. The pole pieces are spaced to form gap 25, which comprises the path of the passage of electron stream 26 between the ferromagnetic material.

Electrostatic deflection plates

27 and 29 are positioned downstream of the ferromagnetic material on each side of the generated electron beam. Internal concentric densifier 31, which comprises a configuration conforming to tubular envelope 11 is positioned in the interior thereof downstream of the cores of ferromagnetic materials. External concentric densifier 33, which also comprises a configuration conforming to tubular envelope 11, is positioned on the exterior thereof upstream from the position of internal densifier 31. The internal and external densifiers may be constructed of a material identical to

ferromagnetic materials

21 and 23.

Lenticular collector plates

35 and 36 are positioned downstream from the electrostatic deflection plates. The collector plates are connected to any suitable indicating means, e.g., galvanometer 37.

In operation, the system is positioned in any direction relative to the local magnetic field. Electrons emitted by cathode 13 are accelerated and focused into electron beam 26 along the axis of the tube by electrode 19 positioned in the tube and maintained at a positive potential with respect to cathode 13 by means of a suitable source of potential represented by battery 39. A potential of 400 volts, for example, is applied to electrode 19 with respect to cathode 13. The voltage is controlled and measured by means of potentiometer 43. A magnetic flux is induced into

ferromagnetic masses

21 and 23 by the earth's magnetic field and any other incidental or local magnetic field. Internal and external concentric magnetic densifiers 31 and 33 aid in channeling the magnetic flux into the path of the columnated electron beam. Densifiers 31 and 33 are tubular in configuration, each with an angular end terminating with a plate that overlies

ferromagnetic masses

21 and 23, respectively. Densifier 31 is in physical contact with ferromagnetic mass 21, and densifier 33 is magnetically coupled to ferromagnetic mass 23. Together, the densifiers with their corresponding plates and masses form a dipole antenna which collects and concentrates the magnetic flux to be detected in gap 25. Since the potential on low acceleration electrode 19 is very low compared to the 10,000 volts on anode 38, the electron beam passing through gap 25 is, comparatively, drifting slowly and thus is very sensitive to the modulations imposed thereon by the highly densified received magnetic flux. Since

masses

21 and 23 are connected to low acceleration electrode 19, the extremely high acceleration effect of cathode 38 is not felt until the cathode beam emerges from gap 25. This feature enables the beam to be excited at low velocity resulting in a high amplification factor, and then be accelerated to a high velocity for further amplification. Generated electron beam 26 is bent by the magnetic flux as it passes through gap 25, accelerated to a high velocity by anode 38, and impinged upon

collector plates

35 and 36. Anode 38 is maintained, for example, at a potential of 10,000 volts with respect to the cathode. Deflecting

plates

27 and 29 serve as null adjustors to compensate for the magnetic flux induced into

ferromagnetic masses

21 and 23 by the earths magnetic field. In operation, a potential is'applied to the deflecting plates sufficient to suppress the effect of the earth s magnetic field. This is indicated by impingement of the electron stream equally on both lenticular plates. When a local magnetic field is induced into the system, and the apparatus has been corrected to obviate the effect of the earths magnetic field, the electron stream impinges unequally on both target plates. Calibrated means indicate the intensity of the local magnetic field. The present combination results in a magnetometer having a sensitivity of 0.0001 gamma which is equal to gauss or better.

The lenticular collector plates are illustrated by FIG. 2. The plates may be coated with a fluorescent powder to make the electron beam impinging on the plates visible for easier compensation.

FIG. 3 illustrates a method for deflecting the electron beam to compensate for the earths magnetic field by means of magnetic deflecting coil 45 in lieu of

electrostatic deflecting plates

27 and 29. The coil is energized by any suitable power source, not shown. Coil 45 establishes a magnetic field traversely of beam 26 whereby the beam is deflected in a direction depending upon the direction of the field established by the coil.

FIG. 4 illustrates a method for deflection of the electron beam comprising small permanent magnets producing magnetic field 47 which is mechanically moved with respect to the electron beam.

The Thermistor bridge illustrated in FIG. 5 obviates the need for lenticular collector plate means. Thermrstors are thermally sensitive resistors whose primary function is to show a change in temperatures. The present bridge comprises therrnistors 49 and 51, meter 53, variable resistor 55, resistor 57 and a power source, e.g., battery 59. After the system has been adjusted to a zero reading, and in the presence of a local magnetic field, the generated electrons impinge unequally on the

thermistors

49 and 51 which unbalances the bridge thereby allowing a measurable flow of current as indicated on calibrated meter 53.

FIG. 6 illustrates an alternative of FIG. 1 wherein thermocouple means 61 and 63, connected to a suitable indicator, designated 65, are utilized as the target means.

Although I have described my invention with a certain degree of particularity, it is understood that the present disclosure has been made by way of example and that numerous changes in the details of construction and combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention.

Having described my invention, I claim:

1. An electron beam magnetometer having a sensitivity equal to 10' gauss or better, comprising an evacuated envelope, cathode means encompassed therein for generating an electron beam, low potential focusing anode means positioned downstream of the cathode means, a first magnetic flux receiver and densifier means connected to a first ferromagnetic mass, and a second magnetic flux and densifier means magnetically coupled to a second ferromagnetic mass, means between said masses defining a narrow gap for the passage of said electron beam, said ferromagnetic masses connected to said focusing anode, deflection means located downstream of the magnetic flux densifier means, target means connected to suitable indicating means located downstream of the deflection means, relatively very high potential accelerating anode means located adjacent the target means, all of said component operably connected to a suitable power source.

2. An apparatus in accordance with claim 1 wherein the deflection means comprises a pair of electrostatic deflection plates located in the interior thereof.

3. An apparatus in accordance with claim 1 wherein the deflection means comprises an energized coil positioned on the exterior of the envelope.

4. An apparatus in accordance with claim 1 wherein the deflection means comprises an external magnetic field positioned adjacent the envelope.

5. An apparatus in accordance with claim 1 wherein the target means comprises a pair of lenticular plates.

6. An apparatus in accordance with claim 1 wherein the target comprises a pair of thermistor means connected to a suitable power source and indicator.

7. An apparatus in accordance with claim 1 wherein the target comprises thermocouples means connected to a suitable indicator.

Claims

1. An electron beam magnetometer having a sensitivity equal to 10 9 gauss or better, comprising an evacuated envelope, cathode means encompassed therein for generating an electron beam, low potential focusing anode means positioned downstream of the cathode means, a first magnetic flux receiver and densifier means connected to a first ferromagnetic mass, and a second magnetic flux and densifier means magnetically coupled to a second ferromagnetic mass, means between said masses defining a narrow gap for the passage of said electron beam, said ferromagnetic masses connected to said focusing anode, deflection means located downstream of the magnetic flux densifier means, target means connected to suitable indicating means located downstream of the deflection means, relatively very high potential accelerating anode means located adjacent the target means, all of said component operably connected to a suitable power source.